The invention relates to a camshaft adjuster for adjusting and fixing the phase position of a camshaft of an internal-combustion engine relative to the crankshaft, with a driving wheel connected to the crankshaft in a rotationally locked way via a suitable drive, with a camshaft-fixed driven part, which is mounted on a camshaft or on an extension of the camshaft and which is driven by the driving wheel, with the phase position of the driven part being adjustable relative to the driving wheel within a certain range of angles.
In internal-combustion engines, camshafts are used to activate the gas exchange valves. The camshaft is mounted in the internal-combustion engine such that cam followers, for example, cup tappets, finger levers, or valve rockers, contact cams mounted in the engine. If the camshaft is set in rotation, then the cams roll on the cam followers, which in turn activate the gas exchange valves. Thus, the position and the shape of the cams sets both the open period, as well as the amplitude, but also the opening and closing times of the gas exchange valves.
Modern engine concepts are directed towards designing the valve drive to be more variable. On one hand, the valve stroke and valve open period should be variably configurable up to the complete deactivation of individual cylinders. For this purpose, concepts, such as switchable cam followers or electrohydraulic or electric valve drive actuators, have been provided. Furthermore, it has been shown to be advantageous to be able to influence the opening and closing times of the gas exchange valves during the operation of the internal-combustion engine. It is also desirable to be able to influence the opening or closing times of the inlet or outlet gas exchange valves separately, in order, for example, to be able to set a defined gas exchange valve overlap. Through the targeted setting of the opening or closing times of the gas exchange valves as a function of the current characteristic field range of the engine, for example, of the current engine speed or the current load, the specific fuel consumption can be reduced, the exhaust-gas ratio can be positively influenced, and the engine efficiency, the maximum torque, and the maximum power can be increased.
The described variability in the gas exchange valve time control is implemented through a relative change of the phase position of the camshaft to the crankshaft. Here, the camshaft is usually in direct driven connection with the crankshaft via a chain, belt, or gear wheel drive. A camshaft adjuster, which transfers the torque from the crankshaft to the camshaft, is mounted between the chain, belt, or gear wheel drive and the camshaft. Here, this adjusting device is embodied such that during the operation of the internal-combustion engine, the phase position between the crankshaft and camshaft is maintained reliably and when desired, the camshaft can be rotated into a certain range of angles relative to the crankshaft.
In internal-combustion engines with a camshaft for the inlet and outlet valves, these valves can each be equipped with a camshaft adjuster. Therefore, the opening and closing times of the inlet and outlet gas exchange valves can be shifted in time relative to each other and the overlap of the gas exchange valve times can be set as desired.
The basis of modern camshaft adjusters is located in general on the drive-side end of the camshaft. It comprises a driving wheel fixed to the crankshaft, a driven part fixed to the camshaft, and an adjusting mechanism transferring the torque from the driving wheel to the driven part. The driving wheel can be configured as a chain, belt, or gear wheel, and is connected to the crankshaft in a rotationally locked way by means of a chain, a belt, or a gear wheel drive. The adjusting mechanism can be operated electrically, hydraulically, or pneumatically.
Electrical adjusting mechanisms are constructed as so-called three-shaft drives. Here, a first shaft (the driving wheel) is in connection, via a linkage, which is driven by means of a second shaft (the adjusting shaft), with a third shaft (the driven part). The adjusting shaft of the linkage is driven by means of an electric motor. Planetary gears, internal eccentric gears, double internal eccentric gears, shaft gears, or wobble-plate gears, for example, are conceivable as the linkage.
In hydraulically operated camshaft adjusters, one differentiates between so-called axial-piston adjusters and rotary-piston adjusters.
In the axial-piston adjusters, the driving wheel is in connection with a piston via oblique gearing. Furthermore, the piston is in connection with the driven part likewise via oblique gearing. The piston separates a hollow space formed by the driven part and the driving wheel into two compression chambers arranged axially relative to each other. Now, if one compression chamber is charged with a hydraulic medium, while the other compression chamber is connected to an oil outlet, then the piston is displaced in the axial direction. By means of the two oblique gearings, this axial displacement creates a relative rotation of the driving wheel to the driven part and thus of the camshaft to the crankshaft.
In a rotary-piston adjuster, the driving wheel is connected in a rotationally locked way to a stator. The stator and the driven part are arranged concentric to each other. The radial intermediate space between these two components accommodates at least one, but usually several, hollow spaces spaced apart in the circumferential direction. The hollow spaces are bounded in a pressure-tight way by side walls in the axial direction. A vane connected to the driven part extends into each of these hollow spaces. This vane divides each hollow space into two compression chambers. Through targeted connection of the individual compression chambers with a hydraulic-means pump or with a hydraulic-means outlet, the phase of the camshaft relative to the crankshaft can be set or maintained.
To control the camshaft adjuster, sensors detect the characteristic data of the engine, such as, for example, the load state and the engine speed. This data is fed to an electronic control unit, which, after comparing the data with a characteristic data field of the internal-combustion engine, controls the adjusting motor of the camshaft adjuster or the inflow and the outflow of hydraulic means to the various compression chambers.
A camshaft adjuster for adjusting and fixing the phase position of a camshaft of an internal-combustion engine relative to its crankshaft according to the state of the art is known from DE 101 61 701 A1. In this publication, a driven part is fixed to a camshaft by means of a central screw. The driven part is arranged concentric to the driving part. In the radial intermediate space between driving wheel and driven part, several hollow spaces are formed, which are closed in a pressure-tight way by side walls in the axial direction. Vanes fixed to the driven part project into these hollow spaces, whereby two compression chambers are formed in each hollow space. The driven part is fixed with the help of a central screw, whereby the driven part is screwed onto the camshaft in the axial direction. The connection is established with a frictional lock through the axial force of the attachment means, which act upon a clamping surface arranged perpendicular to the axial force between the camshaft adjusting unit and the camshaft. The centering of the camshaft adjuster to the camshaft is realized through a complementary connection with radial play.
This actually good solution brings along the disadvantage of an increased axial structural space requirement due to the screw head. Because certain distances between the engine and chassis must be maintained in vehicles for reasons of safety, it is desirable to keep the axial structural space requirement of the camshaft adjuster to a minimum.
Furthermore, in this solution a small eccentricity due to the centering play between the camshaft adjusting unit and the camshaft must be taken into account.
Through the frictional connection of the driven part to the camshaft by means of the central screw, additional stresses are fed into the driven part and the camshaft. To reduce these stresses, in one embodiment, between the driven part and camshaft, there is a sleeve provided with a friction lining, whereby the stress is reduced but not sufficiently overcome.
Furthermore, solid axial clamping surfaces and threading in the camshaft are necessary, whereby considerable additional expense for their production and a high system weight must be taken into account.
Another such camshaft adjuster is described in DE 198 48 607. This is similar to the embodiment from DE 101 61 701 A1. A central screw, which connects the driven part to the camshaft, is arranged in turn within a central bore hole of the driven part. A central valve, which is used for controlling the flow of hydraulic medium to and from the various compression chambers, is integrated in the central screw. In this embodiment, the increased stress on the central valve due to the central screw function has a disadvantageous effect on the device.